I’m just putting two and two together here, so hear me out: if
an iPhone 4 can only make FaceTime calls over Wi-Fi like the iPad
2, could an iPad 2 make a FaceTime call over 3G by connecting to
an iPhone 4’s Personal Hotspot feature which is Wi-Fi?

Yes, this works fine. In fact, while carrying around both my own AT&T iPhone 4 and the Verizon iPhone 4 review unit I got from Apple, I was able to use FaceTime from my iPhone over the Verizon iPhone’s hotspot — and it worked well.